P-n junction device and method of making the same by local fusion



Feb; 4, 1958 Segregation Cbnslanf (K) Relative to Germanium N. HALL 2,822,309 P-N JUNCTION DEVICE AND METHOD OF MAKING THE SAME BY LOCAL FUSION Filed June 20,.1955

Fig. F/ga F4 7. F/ge.

N 4 N 4 p 1 P 6L1 /Ia P 6 "3 /4 P P N N F7 4 Gallium x Arsenic x Ant/many 002- Indium Growl/7 Rate Inches per hbur E F" 6 Q a g- ,o g E J. 58 m 2g Antimony 75 g Intrinsic 6a 9 b I, a E '-P-Type -Type+ F, Gallium k m l I l l l I Growth Rate /nches per Hour /rz venzor Rober'z /V. Ha/l,

conductivity characteristics.

P-N JUNCTION DEVICE AND METHOD OF MAK- IN G THE SAME BY LOCAL FUSION Robert N. Hall, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 20, 1955, Serial No. 516,637

14 Claims. (Cl. 1481.5)

tivity lies intermediate the poor conductivity of insulators and the good conductivity of metallic conductors."

Conduction in semiconductors is primarily electronic, the conduction carriers comprising either electrons or electron vacancies resulting from the movement of electrons. The

degree of conductivity in such semiconductors is substantially affected by changes in temperature and by the amount and type of impurities found therein. Although the present invention may utilize various semiconductors, its chief advantages and widest commercial applications are realized when elemental semiconductors which have a diamond lattice crystal structure and which are found in group IV of the periodic table of elements, such as germanium or silicon, are employed.

These elemental semiconductors have become conventionally classified as either positive, P-type; negative, N-type; depending primarily upon the type and sign of their predominant conduction carriers. With P-type semiconductors, the direction of rectification as well as the 'polarity of thermoelectric, photoelectric, or Hall effect -voltage are all opposite to that produced with N-type .whether a particular semiconductor body exhibits N-type or P-type characteristics lies primarily in the type of dominant impurity elements present in the semiconductor.

Some impurity elements, termed donors, function to furnish additional free electrons to the semiconductor so as to produce an electronic excess material having N-type Such material is denominated as N-type semiconductor. Other impurity elements,

\ termed acceptors, function to remove electrons from the .semiconductor crystal lattice to create material with an excess of positive-holes having P-type conductivity characteristics. Such material is denominated as P-type semiconductor. Antimony, phosphorus, and arsenic, falling in group V of the periodic table,are examples of donor impurities in germanium and silicon, while aluminum, boron, galliun, and indium, falling in group III-B of the periodic table, are examples of acceptor impurities for germanium and silicon. True donors and acceptors such as those enumerated above are characterized in that they add or subtract respectively from the semiconductor lattice structure one electron per atom of impurity independent of temperature above such extremely low temperatures as J United States Patent-O the boiling point of liquid air. Only very small amounts of these impurity elements are normally necessary to produce marked electrical characteristics of one type or the other. Concentrations of some impurities of less than one part per million may be sufiicient.

P-N junction semiconductor units have a zone of P-type semiconductor adjoining a zone of N-type semiconductor to form an internal space charge barrier having a relatively large or broad area as distinguished from the point contact type of device. This junction possesses marked rectifying properties as well as thermoelectric and photoelectric properties. A semiconductor having a region of one conductivity type adjoining two regions of opposite conductivity type to form two P-N junctions can be used to make a three terminal amplifying device known as a transistor.

In the construction of such broad area or P-N junction devices, problems often arise because of their fragility and difliculty of assembly. In some types of devices this problem is minimized by first creating the junction Within an ingot of monocrystalline semiconductor material and then cutting out portions of the ingot containing one or more P-N junctions for use as P-N junction devices. Such methods, however, require careful control of the process by which the activator impurities are selectively added to difierent portions of the semiconductor crystal.

It is, therefore, a primary object of my invention to provide an improved method which forms a P-N junction an improved method for producing transistors having higher frequency ranges than heretofore available.

In accordance with my invention, a monocrystalline semiconductor body is prepared with both acceptor and donor activator elements therein in relative quantities so that one type of activator element predominates to give the semiconductor body initial overall characteristics of one conductivity type. The dominant activator is chosen to have a lower segregation coeflicient than that of the other activator of the opposite type. This segregation coefficient is the ratio of the activator impurity per unit mass in solid phase to that per unit mass in the adjoining liquid phase portion in a growing crystal of semiconductor material. To form a junction within a semiconductor body so prepared, a portion of it is melted and then allowed to cool and recrystallize. Due to the difierence in the segregation coefiicients of 'thedonor and acceptor activators, the recrystallizing portion rejects more of the dominant activator element having the low coeflicient than it does of the other activator element so that the recrystallized portion assumes the conductivity characteristics corresponding to the type of conduction carriers produced by the activator element having the high segregation coeflicient. Since the recrystallization starts or grows from the cooler portion of the melted semiconductor material adjacent the unmelted portion of the semiconductor body, a P-N junction between a zone of the original conductivity type and the converted opposite conductivity type is thus created.

In accordance with a further feature of the invention, the relative and absolute amounts of donor and acceptor impurities within the starting semiconductor ingot are chosenso as to take'advantage'of-thefactthat the'segre- 'gation coefiicients of certain impurity activators are strongly dependent upongrowthrate. With'theamounts of impurity activators so chosen, =the melting and recrystallization is performed, as before, with the "exception that the dependence or "segregation coeffi'cient upon growth rate causes 'a'seeond reversal of conductivity-type semiconductor formeda's the 'ingot recrystallizes. This second reversal of conductivity type results in as'econd P-N junctionbeing formed in the growing ingot. "The resultant ingot having therein two losely spaced P N junctions is useful as a junction 'trans'istor.

The novel features which are believed *cha'racteristie of the invention are set forth in the appended claims. The invention itself, togethe'r with furthe'robjects and advantages therein, may best be understood by relerence to the following descriptionitaken in connection with the 'accompanyingdrawinginwhich:

Figs. :1, 2 and 3 represent successive steps -i n pre- :paring .a :P-N junction device 'in accordance with my invention; EEig. "4 is Pa group *of curves illustrating the tgeneral variation of segregation coefiicie'ntiwith g'row'th irate variation 1: for -:certain exemplary acceptor and donor impurities; Fig. :5 .is a graphical representation of the total segregation of selected impurity activators Iin germanium as a function of growth Irate; Fig. 6 represents .a semiconductive ibarsin which "two P-N :junctions are formed according'to the =invention;lFig. 7 rrepresents -an alternating embodimenteof the device :ofzFig. 3,iand"Fig. 8 illustrates a junction transistor made in "accordance with mytinvention.

Referring Tn'ow 'to .Fig. l, a bar :1 of :germanium is ishown-tlrereinlbefore the, junction :i'ssformed. This bar contains both an acceptor impurity element and. a donor impurity elementuniformlydistributed therein, one of the-activatorelements predominatingso as to give the entire bar either P-type .or N-type condition characteristics corresponding to the type of conduction-carriers supplied by the dominant activator element. Such abar may be extractedfroma monocrystalline ingot grown by the seed crystal withdrawal method of Czochralski described, for example, in an article entitled Preparer tion ofgermaniumsinglecrystals, by Roth and Tay- "lor, in The 'Proceedings of the .IRE, vol. 40, pages 1338 to 1341, November .1952, or the zone leveling technique described, torexample, in an-article -entitled ".Principlesof zone melting,-by W. Pfann, onpage 747, vol. 194, Journal-of .Metals,.]uly .1952. --In the specific example indicatedzinFig. .1, the dominant activator is indium, which,.;since it-is an acceptor activator, furnishes positive conductioncarriers .or .positive holes -togive the germaniumLbody .P-type' conductivity characteristics. The donoractivator in.this=case is:arsenic which, because 'it furnishes free electrons to the semiconductor body, would make .it 'N-type except .for the larger number of opposite-type:conductionacarriers supplie'd by the acceptor activator. vWhile variouscactivator elements may be selected, ."itiisrimportantthat the dorninant-type activator have a substantially .lower -.segreg-altion coefiicient, whichis the casetinvtheexample :given Since arsenic has acoeflicientmany timeshigher than that of indium. As previously-stated,this-segregation-coeflicient is defined as'the ratio of impurity atoms per unit mass in solid phase tothose in r-thetadjoining materialper unit-mass in-l-iquidyphase inwagrowingcryst-al of semiconductor material. Thus, ass-a crystal ofisemiconductor material is ;grown from a 1melt=ofigermanium -.containing traces of both high ar-id 510W. segregation'coefiicient activator elements, a relatively :large percentage of the highcoefiicienttelcmentds:assimilatedrin the 'growing crystal and a relatively small percentageeof. the llow coeificient element is assimilated. For making junctions 4 in accordance 'withmy invention, "the-'semiconductormaterial of the bar 1 is prepared with the low segregation coeificient, dominant activator and the high segregation coefficient, opposite type of activator, both substantially 5 uniformly dispersed or difiused within the semiconductor body. One method of ,preparing such material is described in greater detail in a subsequent paragraph.

As'further shown in Fig. 1,6one zend,tin:this -casethe upper end, of theger'maniumbar 1 is heated until that end melts. Various means of heating 'or heattreatment may be employed but it :is essential :that ithere be a temperaturegradient in thegermanium body 1 so that a portion of the germanium body is heated to a temperature high enough to 3produce-fusion aor melting while another portion remains below the melting point. Local fusion, such as produced by the flame of an oxyhydrogen torch 2 directed at one end of the bar-shaped germanium body 1, is illustrative'of a simple way 'for producing such a gradient. Alternatively, local fusion may be caused by induction heating. Theheating-is preferably carriedon in 'an atmosphere of nitrogen or other inert -gas to prevent oxidation. 'The process may also be carried out within a vacuum, if desired, or ina reducing atmosphere.

.After the heating, as irn'iicate'd 'in 'Fig. "2, 'a portion 3 of 'the germanium -body remains substantially unaf- 'fected:byithe heating process -and=retains'its P-type char- :acteristics. However, the melted portion which is allowed ;to :recryst-allize "acquires N type characteristics 'be- 30 70311883118 acceptor activator has been largely selectively rejected due-Ito its .lo'wsegregation coefficient, leaving the donor activatori'asithe-:dominantaactivator element in that port-ion- 'of thasemiconductonbody.

Thezmeltedportion l 'of the semiconductor bodygrows 5 from :the interface between the :solid semiconductor "of the lower ,portion :3 and :the .liquid semiconductor of .the portion 4. Thex-loss of: heat fromthefmelted portion :to 'the'iSOII ld portion is Linherently greater 1 than "the loss to the surrounding atmosphere, and hence no :special .means are required to maintainrthe temperaturedifieren'tial required for directionalacooling. -Because of the surface tension of the molten ma-teriahthe molten droplet .of germanium remains vin place without flowing awayio fb'ecome.physically'dissociated from the unmelted portion 45 .3. However, due to the llowsegregation coefficient of the .indium and due to the diiference in segregation coeflicients of the indium and .the arsenic, :a much greater lproportionzof the:suspended:arsenic thansof the suspended lindium EiS assimilated ;in ':the .growing .crystal. Thus, tdespite:the-preponderance "of-indium in the melted portion at, :the arsenic :becomes :the dominant activator element .in the 1- recrystallized r portion and the "rejected impurity elementsiwithinathe: portion '4are increasingly con- ;centrated in the still (molten portion as the liquid-.to-solid .interfacemovesnowa-rd lthe upper=endtof the germanium :barrl.

*Sitrceiheabsolute values of the segregation coefiicients Ifor -b'oth arseni'c and indium in germanium are both very small with respect to unity, the amount of arsenic and indiumwithinthe recrystallized portion d is much smaller :than in the unmelted portion -3 of ingot -1. However, since the 1 resistivity of both unmelted portion 3 and re- -crystallized portion T4 of ingot l depends, not upon the absoluteamouritof impurities present, but upon an-excess 65 :of onettype' a'etivator over the other type, the relativeand "absolute 'resistiviti'es of regions 3 and 4 may be regulated andcontrolled "merely by the selectiomof the relative and absolut'e amounts of :donor and acceptor impurities 'in :barfil.

The =transition plane at which the conductivity of -the krecrystallized portion becomes N-type constitutes a P -N 5 :junction between thetwo zones. This P-N-junctionpos- 1sessesfmarked:rectifying,photoelectric and thermoelectric reflEects. :Sin'ce thisiPiN junction is located 'at the junction between the fused and unfused portions of bar 8, it may be readily located physically by the change in the shape of the bar due to fusion.

Referring now to Fig. 3, a completed P-N junctiondevice 4 is shown in which electrical connections are made to the ends of the bar representing respectively the ends of the P-type and N-type zones, such as soldering conductors 7 thereto. The P-N junction semiconductor device so formed is a two-terminal device, suitable for uses such as electrical current rectification.

It is important that the activator elements and the relative quantities of each be properly selected in order that the semiconductor body for use in making P-N junctions in accordance with my invention can be correctly prepared. Reference is accordingly made to Fig. 4 which shows the variation with crystal growth rate of the segregation coefiicients for donor activators antimony and arsenic and for acceptor activators gallium and indium in germanium. The growth rate increases when the temperature gradient within the liquid region is decreased in the process of growing a monocrystalline germanium ingot by withdrawing a seed crystal from a molten mass of germanium to which impurity elements have been added. The curves show that the segregation coelficients differ very widely and that the segregation coefiicients of certain impurity activators increase as the growth rate increases.

Referring still to Fig. 4, it may be seen that if indium is to be employed as the acceptor activator and arsenic as the donor activator, that at any growth rate the proportion of the total arsenic in the melt assimilated in the growing crystal is many times greater than the proportion of the total indium in the melt assimilated in the growing crystal. Therefore, to obtain the preponderance of indium necessary to give the grown crystal overall P- type conduction characteristics, many times more indium than arsenic must be supplied to the melt. Thus, because each atom of arsenic donor element supplies one electron, and each atom of indium acceptor supplies one hole or positive carrier, the ratio of indium to arsenic by weight in the grown crystal must be greater than the ratio of their respective atomic weights. Since the segregation coefiicient of arsenic in germanium is approximately 50 or more times that of indium, the atomic weight ratio in the melt must be increased by more than that factor. This principle governs as well when the invention is practiced with other donor and acceptor activators.

In Table I below, there are listed the equilibrium segregation coetficients of various donor and acceptor activator impurities in germanium and silicon respectively.

TABLE I Equilibrium segregation coefiicients G6 Si Referring to Table I and to the description of the invention set forth hereinbefore, it is evident that a P-N junction may be formed in a semiconductor bar with any combination of donor and acceptor impurities having substantially different segregation coefficients. Thus, all

aluminum make extremely close control necessary. Other combinations having greater differences in segregation coefiicients are preferred. In using any chosen donor-ac ceptor pair to practice the invention, it is only necessary that the activator having the lower segregation coeflicient be in molar excess in the original bar.

The limits between which the relative amounts of selected donor and acceptor impurities added to a semiconductor melt from which a monocrystalline ingot is formed to produce semiconductor bars within which P-N junctions are formed by local fusion are as follows:

(1) In one limiting case, the low segregation coeflicient, dominan t'impurity activator within the monocrystalline bar before fusion is exactly electrically compensated by the high segregation coeflicient impurity activator.

(2) In the other limiting case, the high segregation coefiicient activator impurity in the recrystallized portion of the ingot, after fusion, is exactly electrically compensated by the low segregation coefficient impurity.

Between these two limiting cases, all relative proportions of the selected donor and acceptor activator impurities will produce, after fusion, an ingot having a first region having the conductivity characteristics of the low segregation coeflicient impurity, and a second region (the recrystallized region) having the conductivity characteristics of the high segregation coefiicient impurity. These two regions are, of course, separated by a P-N junction.

The first limiting condition is expressed mathematically by the expression where K is the segregation coefiicient; the subscript (a) refers to acceptor impurity; the subscript (d) refers to donor impurity. This relationship applies to the case when the acceptor impurity is initially dominant and has a lower segregation coefficient than the donor impurity as, for example, when the impurities used are'indium and arsenic. In this case the relationship states that the number of atoms of acceptor activator which are present in the once-processed bar before local fusion are equal to or greater than the number of atoms of donor activator.

The converse limiting case, when the donor activator impurity is dominant in the unfused bar and has the lower segregation coefficient, as, for instance, when the chosen activators are antimony and gallium, is given by the relationship:

The second limiting condition is expressed, in the case when the acceptor impurity has the lower segregation coefiicient by the expression:

This expression means that the number of atoms'of donor impurity entering the recrystallized portion of the bar is equal to or greater than the number of acceptor impurity atoms entering that portion. The converse limiting case, when the donor activator has the lower segregation coefiicient, is given by the expression:

to the original melt may be expressed as:--

Win MOlWlLg V f 1 den where the acceptor has the :lower segregation-.coefiicient, and i Mame, K Win, :Molwtn K311 'Molwt .fi m hdolwt.g (fi where the donor has the .lowe'r segregation 'coefiicien't. In the above equations, the terms we, 7 Wm W'sand W represent the'relative weight proportions of donor and acceptor activator-impurities which areop'erative in producing at least one P-N junction ma-semiconductor bar by local'fus'ion. It shouldbe noted,however,'thatthis iratio refers to' the additions of impurityactivat'ors'to the originalvmelt from which the semiconductor ingot is produced. The ingot is then cut "into small bars and local fusion practiced upon individual bars.

"The absolute amounts of donor andacce'p'tor activator impurities which maybe usedtin practicing the invention are not critical and the method is operative -from the smallest measurable additions up to the maximum solid solubilities of theva'rious elements'in' germanium and silicon. The particular absolute amounts of "activator impurities usedis a matter of design consideration and is governed by the conductivities desired in "the resultant P-N' junction devices.

As a practical matter,however, it is'desirable that the absolute values of donor and acceptor impurities 'be chosen Within the "limits which will cause the .majority conduction carrier excess in the initialbar upon which local fusion is practiced,'and thetmajority conduction carrier'excess in the'recrystallizedsemiconductor droplet to be within the range useful in electronic signal translating devices. The preferable rangeswithin which the selected activator impurities should be addedto a melt-from which the initial semiconductor bar upon which local fusion is practiced are governe'd by the following boundaryconditions.

Firstly, the preferable minimum "addition of each impurityshould be at least as great -as'that'amou'nt as would cause, in the absence of other-impurities, the initially grown,-unmelted bar to have 'a carrier concentration of theorder of 10 carriers per cubic'centirneterof semiconductor, corresponding to a resistivity-of approximately O ohm-centimeters.

Secondly, the preferable *maximum addition of "each 'impurityshould be no greater than that amount-which would cause, in the absence of other impurities, "the useful portion of the fused and recrystallized portion of the semiconductor (excluding the'extreme tip end thereof) to have a carrier concentration of the order of carriers per cubic centimeter of semiconductor,corresponding to 'a resistivity of approximately 0105 ohm-centimeter.

These preferable ranges are set forth in Table-ll below. Certain impurity activators, however, have segregation coefiicients so low...that additions :ofsu'ificient amounts thereof to a semiconductor melt to satisfy the second boundary condition, above, render thegrowth 0f satis factory crystals extremely 'difiicult. 'Eorthese impurities, marked by an asterisk in' Table II, the upper';limit of the preferable ,rangehasbeen set at arbitrary values at which satisfactory crystals may readily be grown from the initial melt.

"TABLE-II Absolute values of additions of impurity activators To germanium (per To silicon (per 100 gm.) 100 gm.)

These amounts, for the purposes of this application, are referred toas 'factivator quantities;of-,the several impurities.

:IlLWlll; be appreciated, however, ,that within'theseranges, the impurities must be selectedintrelative proportions as set forth-inEquations 3 above,*so that theimpurityhaving ithevlower segregation coeflicient dominates the electric-al characteristics oftherinitial semiconductor bar before local ,fusion. 7

In one specific-exampleof the invention, ahigh back voltage,PeNjunction-typediode having a strongly P-type zoneand weaklyvN-typezonetis made bypreparing a melt consisting of ,63 grams of thigh purity germanium having less than 10 ratornsvof impurities per cubic centimeter thereof, corresponding to *aresistivityof above 20 ohmcentimeters, 7.0.milligrams of indium'andl4 micrograms of-arsenic. yAmonocrystalline-ingot was then-grown during solidificationof the melt by .the-well-known seedcrystal withdrawal; technique generally known as the ,Czochralski atechnique. Theiresulting solidified monocrystalline ingot .is1thenislicedilongitudinally by a diamond saw into wires a fewinches,longandabout 0.030 inch wide and and thick. I'he tip .ofztheiwire first grown is then heated for aaboutad :seconds using the reducing portion of the flame of an oxyhydrogen torch (adjusted to very small oxygen content) :in order just barely to melt the tip. Thezhea'tjs removed andthezmelted tip allowed .10 cool and recrystallize,ztherehy to form :P-N ljunction 6 at-thc boundarymfrthennmelted zone ,4. The wire-is then cut about0.050.inch below:the junction 6 to .provide theP-N junctioniunit oflFigJ2. Nickel leads .7 arezthensoldered with :a conventional tin alley ;solder 13 :to the vopposite ends rof thisfiPe'N ujLIIlClilOIl unit 'at aboutz250 'C. toform theldiode-ot Fig.3. tReferring to Table :Labove, it .will be appreciated that such diodes having .:an:initially P- type region randsarrecrystallized N+type region may :also be formed' by-usingotheri'combinations :of 'atlow segregation coefiicient acceptor and a ?higher segregation :coetficient donor in :proportions specified -by Equation 3(a) above, and in activator'quantitics-as specified in Table II above. Such-other cornbina'tions' are, for example, phosphorus-aluminum, phosphorus-gallium, and antimony-indium in germanium; also, phosphorus-aluminum, phosphorus-gallium, phosphorusdndium, =-and arsenic-aluminum, -arsenic-g-allium, arsenic-indium, antimony-aluminum, antimony-gallium, and antimony-indium in silicon. Diodes-'df tliistypewvere foundtopass areverse leakage current of only about 0.2 -milliampere at =10O volts while passing-aforward' current ofabout -14 milliamperes at +1 volt.

In another specific example of the invention, a high forward currentdiode having 'astrongly N-type zone and a weakly P type zone'is madeby preparing a melt consisting of 20 grams of high purity germanium having "less 'than10 dmpurity atoms per cubic centimeter cor- :responding'to arresistivit-y-of above ZOohm-centimetcIs, :150 milligrams of antimony and 0.5 milligram of gallium i-and-thenffollowing the same procedures of crystal formation, local fusion,recrystallization and electrode connec- 'tion set Eforth aboveinconnectionwith the arsenic-indium impregnated "germanium "melt. The resulting diode, shown in Fig. 7, has reverse conductivity characteristics for corresponding zonesofthe diode shown in Fig. 3 and typically passes aboutl milliarnpere reverse current leakage at =volts and about 50 milliamperes forward current at +1 volt. Referring-to TableI above, it will be'appreciate'd 'that=such-germanium diodes having aninitially'N-t-ype-region and arecrystallized P-type region may also be formed by us'ingothercombinations of a low segregation coefiicient'donor and-a higher segregation coefficient acceptor in proportions-specified by Equation 3(a) above, and in activatorquantit-ies as --specified in Table II above. Such other combinations for-germanium orator-example, boron-phosphorus, boron-arsenic, boronantimonygaluniinum arsenic; and'gallium-arsenie.

In yet another specific example of the invention, a high forward current diode was prepared by preparing a melt consisting of 27 grams of silicon having less than 10 impurity atoms per cubic centimeter thereof, which impurity corresponds to a resistivity in excess of 20 ohm-centimeters, 100 milligrams of chemically pure antimony, and 29.8 milligrams of a master alloy of the same high purity silicon as above, and 0.08% by weight of chemically pure boron, and then following the same procedures of crystal formation, local fusion, recrystallization and electrode connection as set forth in the previous examples. The resulting diode possessed characteristics similar to those of the other examples. Referring to Table I above, it will be appreciated that such silicon diodes having an initially N-type region and a recrystallized P-type region may also be formed by using other combinations of a low segregation coefficient donor and a higher segregation coefficient acceptor in proportions specified by Equation 3(a) above, and in activator quantities as specified in Table II above. Such other combinations for silicon are, for example, boron-phosphorus and boron-arsenic.

In growing the ingot or crystal of semiconductor to be subsequently employed for making P-N junctions by local fusion the temperature of the melt is kept constant as the growing crystal is withdrawn, or at least sufiiciently constant so that the relative change in segregation coefficients of the arsenic and indium due to temperature change is not so great as to change the conductivity type of the growing crystal. In my copending patent application Serial No. 304,203, filed August 13, 1952, now abandoned, and assigned to the assignee of the present invention, a method of producing junctions by changing the rate of growth to thus utilize the relatively different activator segregation coefiicient changes with temperature is disclosed and claimed. In the embodiment of the present invention described with relation to Figs. 1, 2 and 3, both in the preparation of a semiconductor ingot or crystal and in the subsequent preparation of a single P-N junction in the crystal, it is the difference in the segregation coefiicient and not their relative changes with growth rate that produces the desired efiect.

The dependence of segregation coefficients of certain activator impurities may, however, be utilized in producing semiconductor devices having therein two P-N junctions in accord with another feature of the present invention. In Fig. 5 of the drawing there is shown a graph of total segregation of selected activator impurities in germanium as a function of growth rate. The curves in the graph of Fig. 5 indicate the total amount of typical donor and acceptor impurities which enter into the growing recrystallized portion of a locally fused germanium bar and represent the product of segregation coefiicient and the total available amount of impurities available to enter the growing crystal. The curves of Fig. 5 are specifically representative of segregation of antimony and gallium in germanium and may be derived from the curves for antimony and gallium of Fig. 4 by adjusting the total available amounts of antimony and gallium so that the product of available amounts and segregation coefiicients causes the total segregation curves of antimony and gallium to intersect. When the total quantities of antimony and gallium in the original semiconductor bar are so chosen that the curves of total segregation versus growth rate intersect at a low growth rate, for instance, between 0 and 6 inches per hour, it is then possible to produce by local fusion a semiconductor ingot having therein two P-N junctions. Such a device may then be used as a transistor for the generation and amplification of electrical signals.

In Fig. 6 of the drawing, there is shown a semiconductor bar 8 including two PN junctions formed in accord with this feature of the invention whereby the dependence of segregation coefficient of certain activator impurities upon growth rate is utilized. As is shown in Fig. 6, a bar of semiconductor material 8 of one conductivity type has a melted .10 and recrystallized portion 8a having a. first zone ,10 of opposite conductivity type and a second zone 11 of the original conductivity type. The device of Fig. 6 may be produced substantially as follows:

A bar 8 of a semiconductor, as, for example, germanium, may be cut from a crystal of germanium containing uniformly dispersed therein a donor activator impurity, the segregation coefiicient of which is substantially dependent upon growth rate as, for example, antimony, and an acceptor activator impurity, the segregation coefficien-t of which is substantially independent of growth rate and higher in absolute value than the segregation coefficient of the chosen donor as, for example, gallium. The relative proportions of antimony and gallium are selected according to criteria, more fully discussed hereinafter, so that the total segregation curves of antimony and gallium as shown in Fig. 5 of the drawing intersect at a low growth rate, for example, between 0 and 6 inches per hour. The relative proportions are such that the donor, antimony, is provided in sufiicient quantity so that the original bar 8, before fusion, possesses N-type conductivity characteristics. The upper portion 8a of N-type germanium bar 8 is then caused to melt by a heating thereof, as, for example, with an oxyhydr-ogen torch as discussed with respect to Fig. 1 while the lower remaining part 9 remains in the solid state. The heating means is then removed and the fused portion 8a recrystallizes from the liquid-solid interface marking the junction between the lower unmelted portion 9 of bar 8 and the fused globule 8a. When the proportions of antimony and gallium'have been so selected that the total segregation versus growth rate curves intersect as in Fig. 5 of the drawing, the first formed portion of the recrystallized globule 8a in the device of Fig. 5 possesses P-type conductivity characteristics. This may readily be seen from the consideration of curve of Fig. 5 wherein it is seen that for slow growth rates, the total segregation of gallium is greater than the total segregation of antimony and a greater number of gallium atoms enters that portion of the crystal than do atoms of antimony, resulting in a molar excess of acceptor (gallium) over donor (antimony) impurities and forming P-type germanium.

The predominance of gallium over antimony in the first recrystallized portion 10 of globule 8a is due to the fact that as the molten globule first begins to recrystallize, growth progresses initially from zero growth rate, increasing as the globule 8a freezes. The growth rate is originally quite slow due to the fact that the rateof recrystallization of the globule depends upon the temperature gradient Within the liquid region adjacent the liquid-solid interface, the growth rate being fastest for low gradients and slowest for high gradients. Since the temperature of the interface is always at the fusion temperature of the germanium-impurity alloy, and the temperature of the unmelted portion 9 of bar 8 does not vary substantially, the temperature gradient across the interface depends primarily upon the maximum temperature within the melted globule 8a atop bar 8. When the recrystallized portion first begins to crystallize, the temperature of this globule is quite high and, accordingly, the original growth rate is very slow. As recrystallization progresses, however, the temperature of globule, 8a falls rapidly and the rate of growth of the recrystallized portion increases progressively, since asthe temperature of the globule falls, the growth rate increases. When the relative proportions of donor and acceptor activator impurities within bar 8 have been properly chosen so that the point at which the curves of Fig. 4 intersect is a growth rate very close to zero growth rate, preferably below 6 inches per hour, and globule 8a cools at a rapid rate, only a very thin P-type region will have formed before the growth rate exceeds the critical value at which the curves intersect.

As further cooling takes place andthe upper part 11 of the fused globule recrystallizes, the growth rate exceeds the critical value and the remaining portion 11 of the re 11 a crystallized ,globule possesses .Nrtype conductivity characteristics because a greaterlnumber.ofantimony.atoms enter the growing crystal than atoms .of-gallium, causinga molar excessof donor.( antimony) over. acceptor (gallium) impurities within region 11. Theintersection between the original N-typeregion 9 of ingotfiand region 10 forms a P-N junctionfia andthe boundary hetweenP-type region 10 and recrystallized N-type .regio 11 forms a second P-N junction6b.

Since it -is necessary :that the .two P-N junctions of a junction transistorhe located withina very .close distance of one another, it [is evident that the above-described change in growth vrate to cause the formation of two closely spaced P-N junctionsmust beconducted at a very rapid rate. I have found that for-the production of efficient junction transistors,thelocallyfused portion of bar 8 must be cooled at a rate greater than 10 centigradedegrees per second, and preferably of the order .100 to 500 centigrade degrees;per second. If the fusedportion of bar 8 is not cooled at arate greater-than l0,cent igrade degrees per second, even if the relative amounts of donor and acceptorimpurities are properly chosemthe twoP-N junctions so formedwill be separatedhy so great adistance that etfective transistor action does not occur, particularly at high frequencies. According tothe invention, rapid rates of cooling'are,assuredbypracticing local fusion upon individual:transistor-sized bars. Such bars should have a maximum cross-sectional :area of 15 square millimeters. Additionally, only -a1small portion of .thesemiconductor bar, approximately=0.l toSO cubic millimeters of :sem conductonis melted, thusinsuring small thermal inertia and .rapid cooling.

Thus, it may ,beseentha-t an N-P-N junction transistor is simply formed, the adjoining. boundaries 'Of the zones .9 and 10 and-the =zones-10 and 11- defining the P-N junction regions .6a and 6b,-respectively. .Connections are suitably made to .the;r-e spective zones for transistor operat on, the central zone being the base and the end zones 9 and 11 being the emitter. and ,collector zones, respectively. The tip 12 of the upper-zone -11 contains such a high-concentration of; residualactivator elements thattitserves chiefly as a conductive terminal.

While-the particular combination of activator elements described is.-exernplar -y, othercombinations may be employed; so long as the low segregation coefficient activator whichis dominant in'unmelted bar 8 has asegregation coefiicient that ,increases much faster .with increasing growth ratethan the opposite-type activator. This is because it is not possible -to-. attain the ;ne ar ba1ance condi- .tion of Fig. and; yet maintain the impurity with the growthfrate sensitive segregation. coefficient as the dominantiimpurityin original-semiconductor bar 8. :Thus, for example-sinc ath donor:el inents arseni an im ny an phosphorus; all 1 satisfy this condition in both ,germanium andisiliconnreferring:te .Table I,.i m y be seen that the invention-mayh p ae ie with the propenprcp ion i combinationsinaactiuator quantities of boron with arsenic, antimony; or; phosphorus. Such combinations are-effective in either germanium orrsilicon. Additionally, the invention may gbe practiced with the combinations aluminumantimony,aluminum-arsenic, gallium-antimony and gallium-arsenic, in germanium.

In order-that thisfeature of the invention be practiced to form semiconductor units having two P-N junctions therein, therelative proportions of donor and acceptor activator impuritiesmust-be chosenso that the original barB, before localfusion, is dominated by the-activator impuri y v gthegsegregation coeflicient which is lower ina s l t magnitud and whi sh w anubstantial. dependence upon growth rate. lLhave found from experiment that heselerite imar satisfie whenith weigh ratio of donor;to acceptor :activatoradded to the ,melt from which bariS is zforrneldtyariest'between40:5 :and 1.0 times the ratio of their molecular weights times the square of seams 1.12 a th ve e atio o their s ga ien-c efli en s Ibi relationsh p m y b expre s d a iM0lWt.a Ka W -a MOlWlLd 7 K03 Molwtl K -WEMOIWsG -(K, 'Thusj c examp weight at o antimony t g ium in germaniumirom 500to 950 have been found to form NrP-N semiconductor units having .two closely spaced .l lI Qn. thereindefining .a' vp ay 0m10...0001 to {0.003 .inch thick, which thicknesses are especially desirable in transistor devices. Satisfactory N -P-N .units have also ,beenformed using weight ratios of arsenic to gallium in germanium from 6,5 to 9.9. Likewise, useful N P-;N units have .been formed using weight ratios of antimony to boronin siliconranging approximately from 10,QQ .3,O00-

Theahsoluteamounts of. donor and acceptor activator material which may be used in practicing this featureof the invention are governed by the same criteria and fall within the sameranges setforth hereinbefore with respect to thehereinbeforediscussedfeatureof the invention in whichonly one .P -N junction is formed. The preferred ranges .of absolute valuesof donor ,and acceptor vwhich produce 7 the most useful devices .in practicing this feature of the inventionare set forth ,hereinbefore in Table II. ,Ina specific example of this featureof the invention, an N-PN junction-type transistor shown in Fig. 8 having a desirably unusually thin P-type base layer 10a of :the order of 0.001 linchthickis made by preparing a melt consistingof 20 gramsof high purity germanium containing less than .10 impurity atoms per cubic centimeter, and corresponding toa resistivity of greater than 20- ohrncentimeters, l-50.mi lligrams of antimony and 0.20 milligram of. gallinm,;and then following the same procedures of crystal formation, local fusion, recrystallization and electrodeconnection set forth hereinbefore in connection withthe arsenic-indium-impregnated melt. An additional electrode connection to thin :P-type 'base layer 10a is made bylconnecting a lead v.13 to .-the layer 10a by an indium solder contact 14 is fusedito and with the edgeof layerlfla to make a good positive charge furnishing connection therewith. Fused indium contact ;14 may be allowed to overlap P-N junctions 6a'and 6b on eitherside oflayer 10asince theindium-is an-acceptor activator and converts thesurface adjacent region to which @itis fused into .P+typematerial-wherebya rectifying P-N junction connection is made between this indiumjimpregnated zone and the N- type zones 9 and 11 on either side of ,P type layer Lllg. typeof elect-rode connection to a thin P type layer of ;an N -P-,N ,transistor forms a portion .of th subjec ma t o .Pa en .7 5.767.

ln-yet anothenspecific exampleof this feature of .the invention, a melt was'prepared containing .27 grams of silicon ;having less -;t-han 10 impurity .atoms per cubic centimeter thereof, which purity corresponds to a :resistivityrinv excess of 20 ioh-m-fientlme'ters, 99 milligrams :of chemically pure antimony tand29r8 milligrams of :a-master alloyofzthegsa-mehigh pnritysilicon asabove and 0.08% hyzweight :of chemically (pure ibOI'QH. This master alloy was @used in orderito accuratelyseontrol .the yerysmall amount of boron ,inihe :melt. jifhesame procedure'of crystal formation, .local z-fusion, andlrecrystallization was followed ,to =produce=an N-type silicon bar approximately oifl60iinch;long,1approximatelyifl 030linch wide and thick, and having two P N junctions v'thereindefining a transverse EP-Iypeizoneapproximately 0;0005 inch thick. A low resistance-base.contacttwasrmadeno this base zone by soldering an aluminum wire thereto at a temperature 'of approximately 5600" 2C. iLow resistance contacts were then madeP-toitheftwooppose'dN-type zones by-soldering 'theretma-wire (if-99% gold and 1 :by weight-antimony aha temperature of approximately 450 -C. The entire unit was :t-hen :etched :in acstrong acid to remove surface p rities. :Ehe *afinishedadevice was then useful as s high frequency transistor,

Transistors made in accord with the above-described methods may typically have a current gain factor from about 100 to 500 when used in a conventional grounded emitter amplifier circuit, and have very thin P-type layers about 0.001 inch thick. It will be appreciated that the thinner the P-type layer, the shorter the time of response of the transistor device to a change in excitation current. Such thin layers are produced because the rate of cooling during recrystallization is unusually rapid, for example, of the order of 150 C. per second and the ratio of the activators assimilated by the recrystallizing bar differs at a correspondingly rapid rate. If a substantial portion of the entire crystalline ingot grown during solidification from the melt is remelted and recrystallized without special provisions to convey heat away therefrom, the rate of cooling is slower, for example, of the order of C. per minute, so that the various impurities are assimilated at a fairly constant ratio by the recrystallizing ingot so that only a P-N junction results at the border of the remelted region.

Thus, it may be seen that a very important feature of the invention is the attaining of very rapid cooling rates which naturally result from melting only a portion of a very small transistor-sized body of semiconductive material. The attainment of such rapid rates of cooling has two important results. Firstly, as is evident from the above, the very rapid cooling rates which result from the use of small units and partial melting thereof promotes variations in crystal growth rates. As is hereinbefore discussed, the using of the changing growth rates makes possible the formation of two P-N junctions in a semiconductor body which is small and which is only partially fused. If the body were large and no special provisions were made to convey heat away therefrom, or if, for some other reason, cooling did not occur rapidly, it would be impossible to form two closely adjacent P-N junctions which encompass an intermediate semiconductor region with sufficiently high conductivity as to form the base region of an efiicient, operable transistor.

Secondly, the use of rapid cooling rates practically eliminates diffusion of impurity activator atoms from one zone to another and results in the formation of sharp, clearly defined P-N junctions which give improved rectification and signal translating characteristics to the devices so formed. As an example of these improved characteristics, transistors formed by local fusion typically have low frequency current amplification factors (d as high as 0.997. Additionally, such transistors typically exhibit an a-cutoif frequency in excess of megacycles per second and oscillate at frequencies as high as 50 megacycles per second.

While the present invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of producing a junction transistor from a monocrystalline semiconductor body having conduction characteristics of one sign and containing dispersed therein a first activator element for producing dominant conduction carriers of said one sign and a second activator element for producing a lesser number of conduction carriers of the opposite sign, the equilibrium segregation coefficients of said activator element in said semiconductor each being less than unity said first activator element having a substantially lower segregation coeflicient at a low crystal growth rate which increases relative to that of said second activator with increase in crystal growth rate, said first activator and said second activator impurities being present within said body in a ratio of 0.5 to 1.0 times the ratio of their molecular weights times the inverted ratio of their segregation coefiicients in said semiconductor which meth' od comprises fusing only a portion of said semiconducto'f to produce a liquid-phase to solid-phase interface therein and allowing said fused portion to cool at a rate greater than 10 centigrade degrees per second and to rapidly recrystallize from said interface to form a first recrystal= lized zone grown at a relatively low growth rate having dominant conduction characteristics provided by said second activator element, and a second recrystallized zone grown at a relatively high growth rate having dominant conduction characteristics provided by said first activator element.

2. A method of producing a junction transistor which comprises forming a monocrystalline semiconductor body having conduction characteristics of one sign and con taining dispersed therein a greater amount of a first activator element for producing conduction carriers of said one sign and'a lesser amount of a second activator element for producing conduction carriers of the opposite sign, the equilibrium segregation coefficients of said activator elements in said semiconductor each being less than unity said first activator element having a substantially lower segregation coeflicient at a low crystal growth rate which increases relative to that of said second activator with increase in crystal growth rate, said first activator and said second activator impurities being present within said body in a ratio of 0.5 to 1.0 times the ratio of their molecular weights times the inverted ratio of their segregation coefficients in said semiconductor fusing a portion of said semiconductor body to produce a liquid-phase to solid-phase interface therein, and allowing said fused portion to cool at a rate greater than 10 centigrade degrees per second and to rapidly recrystallize from said interface at successively increasing growth rates to form a first recrystallized zone grown at a relatively low growth rate having dominant conduction characteristics provided by said second activator element and a second recrystallized zone grown at a relatively high growth rate having dominant conduction characteristics provided by said first activator element.

3. A method of producing a junction transistor from a monocrystalline semiconductor body having conduction characteristics of one sign and containing dispersed therein a greater amount of a first activator element for producing conduction carriers of said one sign and a lesser amount of a second activator element for producing conduction carriers of the opposite sign, the equilibrium segregation coefiicients of said activator elements in said semiconductor each being less than unity said first activator element having a substantially lower segregation coefficient at a low crystal growth rate which increases relative to that of said second activator with increase in crystal growth rate, said first activator and said second activator impurities being present within said body in a ratio of 0.5 to 1.0 times the ratio of their molecular weights times the inverted ratio of their segregation coefficients in said semiconductor which method comprises fusing a portion of said semiconductor body to produce a liquid-phase to solidphase interface therein, cooling said fused portion at a rate greater than IO centigrade degrees per second and recrystallizing said fused portion from said interface at successively increasing growth rates to form a first recrys-' tallized zone grown at a relatively low growth rate having dominant conduction characteristics provided by said second activator element and a second recrystallized zone grown at a relatively high growth rate having dominant conduction characteristics provided by said first activator element.

4. A transistor made in accordance with the method of claim 3.

5. The method of producing an .N-P-N junction semiconductor unit which method comprises preparing a melt consisting essentially of a high purity semiconductor, and activator quantities of an acceptor activator impurityiand 15 a donor activator impurity, said donor activator .impurity having a segregation coeific'ient in said semiconductor which is substantially lower than that of said acceptor activator, the equilibrium segregation coeificients of said activator elements in said semiconductor each being less than unity said donor and said acceptor activator impurities being present in relative proportions so that the weight ratio of the acceptor to donor activator is from 0.5 to v1.0 times-the ratio of their molecular weights times the square of the inverted ratio of their segregation coefiicients, growing a monocrystalline ingot having N-type conductivity characteristics from said melt, extracting a small crystal from said ingot, locally melting a surface adjacent region of the extracted crystal and allowing the melted .portion of the crystal to recrystallize to form a first cool ata rate greater than centigrade degrees per second 'andto'rapidly recrystallized zone having P-type conductivity characteristics and a second recrystallized zone :having N-type conductivity characteristics.

.6. The method of producing an N-P-N junction ztransistor which method comprises preparing a melt cons'isting essentially of high purity silicon and an acceptor-donor activator impurities combination for silicon, said .activator impurity combination being selected :from the group consisting of boron-arsenic, boron-antimony, .and boronphosphorus, said donor and acceptor i-being addedinactivator quantities and in relative .proportionsso 'that the weight'ratio of the selected acceptor to the selected donor is from 0.5 to 1.0 times thezratioof their molecular sve'ights times the square of the inverted ratioof theirrsegregation coefiicients, growing a monocrystalline ingothavingrNetype conductivity characteristics from said :melt, extracting a small crystalfrom said ingot, locally melting a surfacezadjacent region of the extracted crystal and allowing :the meltedportion of thecrystal tocool at a rategreater than 10 centigrade degrees per second and to rapidly recrystal- 'lize to form a first recrystallized zone :having :P-itype v ceptorand donoribeingadded in activatorquantities and in relative proportions so that the weight ratio of :the selected acceptor to fthe selected :donor is from 045 ito 1.0 times the ratio-oftheir molecular weights times-the square of the inverted ratio ofitheir segregation coefiicients, :growing a monocrystallineiingot from said-melt, extracting a small crystal from -said'ingot,locally melting asurfaceradjacent region of the extracted crystal and allowing Ithe melted portion of the crystal to cool at-a rate of 100m 500 'centigrade degrees ;per second andtorapidly recrysta'llize :to form a first recrystallized zone'havingP-type conductivity characteristics and a second recrystallizedzone having N-type conductivity characteristics.

9. The method of claim 8 in which the acceptor-donor combination is gallium-antimony.

10. A semiconductor device comprising-a monocrystalline semiconductor body having a first-crystallized region having uniformly dispersed therein activator quantities of an impurity-activator of a first conductivitydnducing *type,

and an impurity activator of a, second conductivity -inducing type, the'equilibrium segregation coefficients of said activator elements in said semiconductor eachbeing less than unity said ,first type activator having a substantially lower segregation coeflicient,which'increases substan- ,tially with increasingcrystal-growth rates, .saidfirst activartorbeing presentin molar excess'over said second activator 16 to cause said first region to be of said first conductivity type, a second recrystallized region having uniformly dispersed'therein-s'aid first and said second activator with said second activator present in molar excess over said first activator causing said second region to be of said second conductivity type and to form a P-N junction with said first region,-a third recrystallized region having therein said first and second activator with said first activator present in :molar excess over said first activator causing said third region ;to ;be of said first conductivity type and to form a P-Njunction with saidsecond recrystallized region, and respective .low resistancecontacts to each of said regions.

11. An vN-P-N junction transistor comprising a monocrystalline semiconductor body having a first crystallized region having uniformly dispersed therein activator quantities ofan acceptor activatorimpurityand a molar excess of a donoractivat-or impurity having a segregation coefiicient lower than :that of saidacceptor impurity and which increases relative thereto with increasing crystal growth rate to cause said first region to have N-type conductivity characteristics the equilibrium segregation coefficients of said activatorelements in said semiconductor each being less than unity, a second recrystallized region having therein saidtdonor impurity and a molar excess of said acceptor impurity to cause said region .to possess P-type conductivity characteristics and to form a P-N junction with said firstrregion, a,third recrystallized region having ,therein'said gacceptor activator impurity anda molar excess of said donor activator impurity to cause said region to exhibit rNtype conductivity -,characteristics and to form a P-N junction with said second recrystallized region, and respective low resistance contacts toeach of said regions.

12. 'Ihemethod of producingan N-P-N junction transistor,-which method comprises preparing a melt consisting essentially of high purity germanium impregnated with antimony =.an,d gallium, said antimony;and gallium being added to the melt in activatorguantities andinmelative proportions of 500 to 950 parts by Weight-of antimony to each part gallium, growing a monocrystalline ingot having N-typeconductivity Vcharacteristics from said melt, extracting a small crystal from said ingot, locally melting a surface-adjacent region of the extracted crystal and allowing the melted portion of the crystal to cool at a rate greater than 10 :centigrade degrees per second and to rapidly recrystallize to forma first recrystallized zone having P-rtypeconductivity characteristics and a second recrystallized zone having Nftype conductivitycharacteristics.

13. The method of producing an N-P-N junction transistor which method comprises preparing a melt consisting essentially of high purity germanium impregnated with arsenic and gallium, said arsenic and gallium being added'to the melt in activator quantities and in relative proportions of 6. 5 to 9.9 parts by weight of arsenic to each part-gallium, growing a monocrystalline ingot having 'N-ty-pe conductivity characteristics in said melt, extracting a small crystal from said ingot, locally melting a surface adjacent region of said extracted crystal and .allowingtthe melted portion of the crystal to cool at a rate greater than 10 centigradedegrees per second to :rapidly recrrystallize to form a first recrystallized zone zhaving ePstype conductivitycharacteristics and a .second recrystallized zone-havingj-Niyp'e conductivity charact risti ss 14. ,The method of producingsan N-P-N junction transistor which-method comprises preparing a melt consist- ,ing essentially of high-purity silicon impregnated with antimony .and boron, said .antimony and boron being added in .activatonguantities and in relative proportions {of 19,000 to 18,0.GOFparts antimony to each part boron by weight growinga monocrystalline ingot having N-IYPe conductivity characteristics from saidmelt, extracting a small crystal from said ingot locally melting a surface j t region of the extracted crystal and allowing the 17 18 melted portion of the crystal to cool at a rate greater 2,583,008 Olsen Jan. 22, 1952 than 10 centigrade degrees per second and to rapidly 2,691,736 Haynes Oct. 12, 1954 recrystallize to form a first recrystallized zone having 2,739,088 Pfann Mar. 20, 1956 P-type conductivity characteristics and a second recrys- Y tallized zone having N-type conductivity characteris- FOREIGN PATENTS tics 510,303 Belgium Apr. 15, 1952 References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS Pfann: Journal of Metals, July 1952, pages 747-753.

2,567,970 Scafi et a1 Sept. 18, 1951 10 

1. A METHOD OF PRODUCING A JUNCTION TRANSISTOR FROM A MONOCRYSTALLINE SEMICONDUCTOR BODY HAVING CONDUCTION CHARACTERISTICS OF ONE SIGN AND CONTAINING DISPERSED THEREIN A FIRST ACTIVATOR ELEMENT FOR PRODUCING DOMINANT CONDUCTION CARRIERS OF SAID ONE SIGN AND A SECOND ACTIVATOR ELEMENT OF PRODUCING A LESSER NUMBER OF CONDUCTION CARRIERS OF THE OPPOSITE SIGN, THE EQUILIBRIUM SEGREGATION COEFFICIENTS OF SAID ACTIVATOR ELEMENT IN SAID SEMICONDUCTOR EACH BEING LESS THAN UNITY SAID FIRST ACTIVATOR ELEMENT HAVING A SUBSTANTIALLY LOWER SEGREGATION COEFFICIENT AT A LOW CRYSTAL GROWTH RATE WHICH INCREASES RELATIVE TO THAT OF SAID SECOND ACTIVATOR WITH INCREASE IN CRYSTAL GROWTH RATE, SAID FIRST ACTIVATOR AND SAID SECOND ACTIVATOR IMPURITIES BEING PRESENT WITHIN SAID BODY IN A RATIO OF 0.5 TO 1.0 TIMES THE RATIO OF THEIR MOLECULAR WEIGHTS TIMES THE INVERTED RATIO OF THEIR SEGREGATION COEFFICIENTS IN SAID SEMICONDUCTOR WHICH METHOD COMPRISES FUSING ONLY A PORTION OF SAID SEMICONDUCTOR TO PRODUCE A LIQUID-PHASE TO SOLID-PHASE INTERFACE THEREIN AND ALLOWING SAID FUSED PORTION TO COOL AT A RATE GREATER THAN 10 CENTIGRADE DEGREES PER SECOND AND TO RAPIDLY RECRYSTALLIZE FROM SAID INTERFACE TO FORM A FIRST RECRYSTALLIZED ZONE GROWN AT A RELATIVELY LOW GROWTH RATE HAVING DOMINANT CONDUCTION CHARACTERISTICS PROVIDED BY SAID SECOND ACTIVATOR ELEMENT, AND A SECOND RECRYSTALLIZED ZONE GROWN AT A RELATIVELY HIGH GROWTH RATE HAVING DOMINANT CONDUCTION CHARACTERISTICS PROVIDED BY SAID FIRST ACTIVATOR ELEMENT. 